Studies on the mechanism of prokaryotic mutagenesis have focused on the roles of the RecA and UmuDC-like mutagenesis proteins. Biochemical assays have revealed that UmuD, UmuD' and the functionally homologous MucA' proteins physically interact with RecA. This interaction may provide a mechanism by which the Umu-like proteins are targeted to lesions in DNA. Two E. coli strains have been constructed that allow the identification of Umu phenotypes based upon a simple phenotypic complementation assay. These strains have facilitated the cloning of three new umu-like operons from R-plasmids R391,R446b and R471a. These tester strains were also utilized to identify several novel plasmid-encoded umuC mutants. In studies on mammalian DNA repair, we determined the subcellular distribution and regulation by UV light of a 127 kDa protein component of a primate UV-damaged DNA-binding (UV-DDB) complex. Structural homologs of this UV-DDB protein were identified in slime mold and rice, and a partial cDNA of a Drosophila homolog was isolated. New evidence supports an important role for the UV-DDB complex in mammalian DNA damage-recognition and repair: One additional Xeroderma Pigmentosum patient ("XP variant") with a defect in UV-DDB activity was identified (4 patients are now known to lack this damage-recognition activity); the recovery of UV-DDB activity is delayed in UV-irradiated cells from XP groups A, D, and C patients, which correlates well with the DNA repair-deficiency found in these cells; and cells from mammalian tissues that do not normally express UV-DDB activity are more UV sensitive than their clonal isolates which have regained the binding activity. In studies on SV40 as a model eukaryotic replicon, we focused on the viral small t antigen. In a purified in vitro DNA replication system, we found that small t inhibits large T antigen-dependent SV40 DNA replication. However, in vivo, small t enhances viral DNA replication. Infection experiments with small-t mutant viruses indicate that this antigen stimulates progression of permissive monkey cells - but not of non-permissive rodent cells - from the G0/G1 to the S phase of the cell cycle, presumably leading to an optimal intracellular environment for viral replication. In rodent experiments, small-t mutants preferentially transformed rapidly proliferating lymphoid cells and appeared unable to transform cells that normally have a low mitotic rate, such as mesothelial cells. In these experiments, we also found that wild-type SV40 induces mesotheliomas in hamsters. Subsequently, we have found that more than 60% of human mesotheliomas contain and express SV40-like sequences.